This invention relates to a process for the adsorptive purification or fractionation of gaseous mixtures using several cyclically reversible adsorbers.
A similar process is disclosed, for example, in DOS [German Unexamined Laid-Open Application] 1,919,557. In that reference (FIGS. 4 and 5), there is illustrated and described a process which operates with four cyclically reversible adsorbers, wherein, in a pressure swing operation, the adsorption phase is succeeded by three expansion phases, a purging phase, and two pressure build-up phases. The first expansion phase takes place in pressure equalization with the first pressure build-up phase of another adsorber; the gaseous fraction discharged during the second expansion phase, operated cocurrently, is used to purge another adsorber; and the gaseous fraction obtained during the third and last expansion phase, operated countercurrently, is withdrawn directly as a residual gas. The gaseous fraction obtained during the course of the purging phase, operated at the lowest pressure, is also discharged as a residual gas. Accordingly, the residual gas is continuously discharged, namely alternatively from the countercurrent expansion phase and from the purging phase. The first pressure build-up phase is conducted in pressure equalization with the first expansion phase of another adsorber, and by the introduction of a purified product gas. The second pressure build-up phase takes place exclusively with a purified product gas.
In this prior art process, voluminous regulating measures are necessary to obtain a constant stream of a product gas. Such regulating measures are necessary because constantly purified product gas, in periodically fluctuating amounts is branched off during the adsorption phase for the pressure build-up stages. A similar control for the residual gas stream, however, is not provided. Thus, it can be of considerable disadvantage, if the adsorbed components, entrained by the residual gas, are used for some commercial application and it is important to maintain a substantially constant flow of the residual gas. The residual gas in the conventional process, however, is obtained with substantial periodic fluctuations. The flow of the volume stream or mass stream of the residual gas decreases substantially during the countercurrent expansion phase. In contrast, after the expansion phase, it substantially increases at the beginning of the purging phase and then decreases again at the end of the purging phase. To compensate for such fluctuations, it is necessary to use buffer tanks, which can assume considerable dimensions and cause correspondingly high costs in initial capital investment.
The problem of compensating for the fluctuations occurring in the residual gas is also posed in pressure swing adsorption processes operating with more than four adsorbers and more than three expansion phases. Thus, for example, in the process of DOS [Unexamined Laid-Open Application] 2,624,346, a buffer tank must be provided for the exiting residual gas. This buffer tank is required even in this process although, with an increase in the number of adsorbers and the resulting increase in the number of expansion phases and the pressure build-up phases, the fluctuations occurring in the thus-obtained gaseous streams are generally diminished. In the process of DOS 2,624,346, operating with nine adsorbers, the first and the second expansion phases follow the adsorption phase, each of these expansion phases being operated in pressure equalization with corresponding pressure buildup phases of other adsorbers. Subsequently, the adsorber undergoes: a third expansion phase yielding a purging gas for another adsorber; a fourth expansion phase operated in pressure equalization with the first pressure build-up stage of the just-purged adsorber; a fifth expansion phase yielding a gaseous fraction introduced directly into the residual gas conduit; a purging phase with the use of the gaseous fraction obtained during the third expansion phase of another adsorber as the purging gas; and the four pressure build-up phases, the first three of which are operated in pressure equalization with corresponding expansion phases of other adsorbers, purified product gas being required for the last of these pressure build-up phases. In this mode of operation, strong fluctuations also occur in the residual gas upstream of the buffer tank, since the total residual gas stream is comprised of volumetrically different gas streams obtained chronologically simultaneously from the fifth expansion phase and from the purging phase of two adsorbers, from the chronologically simultaneous purging phases of the same two adsorbers, and from the purging phase of one adsorber.
Gaseous mixtures purified and/or separated according to the process of DOS 2,624,346 yield valuable residual gas components. For example, raw hydrogen can be purified according to that process and the residual gas obtained therein still possesses considerable heating value. If such a gas is to be used in a subsequent combustion process, it is generally desirable to avoid fluctuations in the supply of the gas. In the case of the fractionation of natural gas according to that prior art process, the residual gas is comprised of a considerable amount of methane and other light hydrocarbons. If it is desired to separate the methane from the other hydrocarbons, it is also desirable to maintain a chronologically constant supply of the residual gas.